The present invention relates to an arrangement for more even current distribution in a transmission line. The arrangement is primarily intended for use at microwave frequencies, and its main application is within the antenna field, above all for feeding so called patch antennas.
In modem communication systems, it is of the utmost importance to reduce the weight, to keep the production cost down and to minimize the electric losses of the antennas used in the system. To this end, use is often made of what are known as patch antennas, in other words antennas which have plane radiating elements, or patches, which have their main extension in two planes.
The patches are often fed via a feed network which comprises open transmission lines arranged in association with dielectric material. The losses in these open transmission lines are due mainly to losses in the dielectric materials and to limited conductivity in the transmission lines, combined with what is known as the skin effect.
The limited conductivity in the conductors combined with what is known as the skin effect in these conductors causes the current flowing in the conductor to be distributed unevenly over the cross section of the conductor in the longitudinal direction of the conductor, the majority of the current flowing in the conductor flowing in small areas concentrated around the outer edges of the conductor. This phenomenon is called current crowding, and increases as the frequency of the current in the lines increases, which makes the phenomenon especially troublesome in the microwave range.
The known art for reducing current crowding in a conductor within the microwave range comprises arrangements in which the conductors and the dielectric material are designed with a V-shaped cross section in the longitudinal direction of the arrangement. This has the effect of reducing current crowding to a certain extent, but has a disadvantage in that mass production of arrangements of V-shaped cross section is difficult and expensive.
Another known arrangement for reducing current crowding in a conductor within the microwave range comprises a conductor with a rectangular cross section in the longitudinal direction of the arrangement, arranged on a dielectric material likewise with an essentially rectangular cross section in the longitudinal direction of the arrangement, where the dielectric material has recesses, xe2x80x9cgroovesxe2x80x9d, on both sides of the conductor. This has the effect of reducing current crowding to a certain extent, but this arrangement is also difficult and expensive to mass produce.
Japanese patent application 3010402 discloses a device for obtaining a microstrip line with small transmission losses, by means of providing a magnetic body in a position facing the conductor, with the interposition of the dielectric substrate. A drawback of this device would be that most magnetic materials are inherently lossy, which means that the introduction of a magnetic body in the device will not lower the losses as much as might be desired. In addition, magnetic materials are expensive to use, since they are not used as standard materials within the electronics industry. Thus, the device disclosed in JP 3010402 will have relatively high losses, and will be relatively expensive to produce.
The object of the present invention is therefore to obtain an arrangement for an open transmission line, with low losses and even current distribution, which arrangement is simple and inexpensive to mass-produce. The arrangement according to the invention is also to be of low weight.
These objects are achieved by means of an arrangement which comprises a first conductive layer, a dielectric layer, and a ground plane, where the first conductive layer, the dielectric layer, and the ground plane have their main extent essentially in the same direction, with the dielectric layer arranged between the first conductive layer and the ground plane, further comprising an object located between the dielectric layer and the ground plane. Said object consists of a dielectric body, or of a body made of a conductive material.
The use of an object made of a dielectric material or of a conductive material allows an arrangement according to the invention to be manufactured using only materials which are standard materials in the electronics industry, thus making the arrangement easy and inexpensive to manufacture. In addition, an arrangement according to the invention will have significantly lower losses than the background art. Also, the use of an object made of a dielectric material or of a conductive material allows the object to be made simply as a protrusion of the above mentioned dielectric layer or the ground layer.
In a preferred embodiment of the invention, said object of a dielectric material connects the ground plane to the dielectric layer.
In a further preferred embodiment, said object of a conductive material is connected only to the ground plane.
These and other advantageous embodiments of the invention will be described in greater detail below.